The present invention relates to film deposition method.
Optical members such as optical lenses, displays and optical communication members are formed at their surfaces with antireflection coating for lessening a loss in the amount of light or the like due to reflection. In the case where the substrate on which such antireflection coating is to be formed is glass, magnesium fluoride (MgF2) is typically used as the surface layer of the antireflection coating. This is because: magnesium fluoride has a refractive index as low as 1.38 and hence is highly effective in antireflection; a film of magnesium fluoride can be deposited easily by vacuum evaporation; such a magnesium fluoride film has sufficient durability if deposited on a substrate heated to about 300° C.; and a like reason.
If the substrate is formed of a plastic, however, the vacuum evaporation cannot be employed because it is impossible to heat the substrate to such an elevated temperature.
In view of this, a method of forming a magnesium fluoride film using sputtering has been disclosed in Japanese Laid-Open Patent Application Publication No. HEI 9-243802 for example. With the sputtering it is possible to form a dense and hard film on the surface of a substrate without the need of heating the substrate because the energy of the film material popping out of the target in the sputtering is higher than that in the case of the vacuum evaporation.
In the formation of a magnesium fluoride film by sputtering, however, fluorine is dissociated from the film material (magnesium fluoride) due to impact of ions when the film material pops out of the target, thus resulting in a fluorine-deficient film exhibiting an increased light absorption on the surface of the substrate. To avoid this inconvenience the aforementioned film-forming method controls heating of the target to a predetermined high temperature to allow the film material to pop out of the target with its molecular state kept as it is. By so doing the dissociation of fluorine from the film material is prevented, with the result that an increase in light absorption is suppressed.
Thus, the film-forming method employing the sputtering must control the temperature of the target to allow the film material to pop out of the target with its molecular state kept as it is.
Such a problem arises commonly in forming not only magnesium fluoride films but also halogen compound films for use as various optical thin films such as antireflection coating and half mirror coating by the use of the sputtering.